Before adding products for your eCommerce shop, we recommend creating any brands and categories you will use. This way, when you are ready to add products, the brands and categories you've created will already be present as options you can quickly apply to each new product.
Add brands via: Sell>Create A Business/Brand
Add categories via: Tools>Create A Category (select "Product" from the component dropdown menu)
Brands and categories you've created will then appear in the respective dropdown menus on the product creation page (Sell>Create A Product):
This gives you and your customers the ability to filter and sort by brand/category on your shop page. See discountcreditcardsupply.com for examples of this in action.